A conventional multi-layer molding nozzle comprises a multiple nozzle in which a nozzle body having a central mouth portion at a tip thereof is provided interiorly with a plurality of concentric resin paths, and different molten resins in the respective resin paths can be cavity-injected from said mouth of the nozzle.
The formation of a synthetic resin molded product into a multi-layer causes the quality of a thin-wall molded product (such as a beverage container) to be further improved by the resin present as an intermediate layer. However, this poses a problem in that because of the provision of the intermediate layer, the shock resistance of the product is lowered or the cost of the product is lowered or the cost of the product increases, as compared with single layer products.
In view of the foregoing, an attempt has been made to use a conventional double or three-layer nozzle to control injection pressure, time, injection timing or the like of the molten resin which forms an intermediate layer so as to injection-mold a product partly having an intermediate layer. However, it has been very difficult to mold an intermediate layer within a predetermined range in terms of resin pressure.
Alternatively, a multiple nozzle is used in which a central path of a multiple resin path is made movable so that orifices of resin paths are selectively opened and closed to control molten resin, as disclosed in Japanese Patent Application Laid-Open No. 60-34819. However, in this case, a special valve device is required to control a flow of molten resin entering the central path passing through the orifice for injection, and in addition, the valve device is mechanically operated, and the nozzle has an extremely complicated construction.